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Risultati finali
Block copolymers have been synthesized and tested for surface adhesion properties and also film properties.
The correlation between molecular weight distribution, particle size distribution, surface properties, film formation properties and end-use properties have been determined.
The AFM images of the investigated samples made of styrene and butyl acrylate showed that all diblock and triblock copolymers have a microdomain structure when pre-heated to 130°C. Spherical domains of styrene were observed as expected for a St/BA composition equal to 15/85.
The surface energy of the latex films decrease when the film is pre-heated to 130°C. This indicates that the microdomains are more exposed at the surface when the temperature is increased.
The viscoelastic properties depend on all the parameters investigated: surfactant, Mn, St/AB ratio and polydispersity.
Based on the prerequisites of a good PSA latex, the studied polymers should ideally exhibit low polydispersities, high polystyrene content and a low molecular weight.
Some novel nitroxides were synthesised and tested. However, the results were not promising and showed that there was no control of the polydispersity or molecular weight distribution.
It was then decided to investigate the use of xanthates (MADIX agents) in polymerization of styrene, alkyl acrylates and vinyl acetate. It was found that:
- O-ethyl xanthates control the polymerization of styrene, ethyl and butyl acrylate, and for the first time vinyl acetate. They exhibit moderate reactivities towards styrene and acrylate propagating radicals. This resulted in high polydispersities (Mw/Mn-2). However, molar masses were controlled and block copolymers of styrene and acrylates were readily synthesized. In the case of vinyl acetate, polydispersities are much lower (<1.5).
- High polydispersities were correlated with transfer constant values close to unity. The evolutions of Mn and polydispersity during MADIX polymerization of styrene, fitted with Muller equations that were established for a degenerative transfer process.
- In xanthates RS(C=S)OR, the R leaving group only affects the first transfer event in the process, in other terms the transfer constant to xanthate, but not polydispersities. The polydispersity is dictated by the PnS(C=S)OR chain end.
- Diblock copolymers of styrene and butyl acrylate were synthesized in solution, in batch and semi-continuous addition conditions.
Emulsions and miniemulsions of nitroxide-mediated polymerizations met with difficulties, arising from the colloidal stability to compartmentalization and exit of the radicals. Nitroxide mediated polymerization has the inherent drawback that exit of the persistent radical species (nitroxides) into the water phase will not only terminate oligomeric species in the aqueous phase but also will allow the loss of control inside the particles.
This disadvantage was overcome with the MADIX process. The influence of MADIX agents on parameters such as rate, entry and exit, and molecular weight distributions was investigated. Styrene, butyl acrylate and vinyl acetate were polymerized n a controlled fashion. Various processes like seeded, semi-batch and ab initio polymerizations were carried out. The synthesis of diblock copolymers of styrene and butyl acrylate was optimised by controlling the reaction conditions.
ABA triblock copolymers comprising styrene and butyl acrylate were prepared for the first time in dispersed media.
This study enabled to show that the MADIX process is by far the most suitable technique to design controlled polymers in emulsion.
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